The present invention relates to methods for holding in position, for the purpose of machining a concave pivoting bearing surface, a roller segment having an external rolling surface with a rounded transverse profile, said roller segment being intended in particular to form part of a motor vehicle transmission joint.
The present invention also relates to a device for holding a roller segment of this kind in position for the purpose of the machining.
The invention relates in particular, but not limitatively, to roller segments intended to form part of a transmission joint comprising three spherical trunnions forming a tripod at the end of one of two shafts connected together by the joint. Depending on the direction of the movement to be transmitted, each spherical trunnion bears, through a roller segment, against one or another of two respective longitudinal rolling tracks provided inside a bowl fixed to the other of the two shafts connected together by the joint. Six roller segments are thus interposed, each between one of the rolling tracks and the respective trunnion. Each roller segment has a concave spherical bearing surface pivoting on the spherical trunnion, and an external toric rolling surface which is able to roll on the rolling track and to be inclined laterally relative to the rolling track, which has a concave cylindrical profile. These sliding homokinetic joints possess the properties of comfort and compactness required for equipping modern motor vehicles. They are described in French Patent Application No. 86 17 044 of the 5th Dec. 1986 now GB 2199113.
The remarkable performance of these joints is to a a large extent due to the hydrodynamic lubrication of the spherical articulation between the spherical trunnion and the concave spherical bearing surface of the segment. This hydrodynamic lubrication practically cancels out the resultant slide drag, while accepting high pressures for the load transfer. The constitution of the extremely thin film of lubricant--taking into account the low speeds of sliding--entails perfect complementarity of the surfaces.
Methods are known for the machining of the concave pivoting bearing surface which enable only an approximate complementarity to be achieved. These methods therefore entail additional stages of grinding in of the two surfaces, one against the other, by a method used in the production of optical lenses. The need for such grinding in is incompatible with the imperatives of cost price and production rates of the motor vehicle industry.
In order that the machining of the pivoting bearing surface may attain the desired precision, it is necessary to hold the segment accurately in position during the machining.
The following technical requirements must in fact be complied with:
(a) Accurate achievement of the radial thickness dimension of the segment, which in large-scale series production permits assembly of joints with minimum clearance without risk of seizing;
(b) Accurate positioning of the axis and/or center of the pivoting bearing surface, which may be slightly eccentric relative to the center of the external rolling surface if it is desired to create the effect of a return of the segment to a neutral position in which the section bearing against the rolling track is situated at a circumferential mid-length of the segment. This effect is obtained if the radial thickness of the segment increases slightly from the section towards each end of the segment.
(c) Where necessary, correction of the concave bearing surface by the application to the segment appropriate forces during the machining so that the effective contact between the trunnion and the concave bearing surface under load will be as extensive as possible, despite the deformation of the segment through the action of the load. French Patent Application 87 13 976, unpublished, teaches that the behaviour of the segment under a real load can be simulated by applying, to the two ends of the segment, tangential compressive forces which are balanced by a force directed radially inwards and applied at circumferential mid-length of the external rolling bearing surface of the segment. Consequently, if the pivoting bearing surface is machined while the segment is thus artificially loaded, the bearing surface of the segment will be exactly complementary to the trunnion when the segment is subjected to its real nominal load.
The aim of the present invention is therefore to provide a method and a device for holding a roller segment in an accurate position appropriate for the machining of its concave pivoting bearing surface, despite the fact that the shape of the roller segment is complex and relatively unfavourable for such precision.